For many years the forest products industry did not concern itself with its future wood supply, since the American forests were seen as being inexhaustable. It was not until well into the second quarter of this century that it became painfully apparent to a few farsighted forest managers that this was not the case. The first commercial tree farm established in 1941 marked a major turning point in attitudes.
Initially cut over land was restocked by leaving individual trees or blocks of trees as a seed source. In many cases the land would become overgrown with brush or undesirable tree species since good seed crops on most coniferous species occur only at infrequent intervals. Later, natural seeding was supplemented by aerial seeding and by hand planting seedlings grown in large nurseries.
Originally, nurseries had to draw exclusively on wild seed supplied by individual collectors. Only in the last 20 years has serious consideration been given to genetic improvement of the forest crop. To date this has largely been implemented by creation of seed orchards based on grafted scions from naturally occurring superior trees. A few second generation orchards are now in production based on trees produced by selective crossing of improved genotypes.
Unfortunately, achieving genetic gain in trees is a slow process because of the long crop cycles necessitated by slow sexual maturation. It has long been apparent that the gain could be accomplished much more rapidly if trees for restocking could be reproduced asexually. Grafting has proved far too slow and expensive for forest restocking, since many hundreds of millions of tree seedlings are planted each year. Reproduction through tissue culture has been an attractive possibility. However, the coniferous trees have been so difficult to reproduce in culture that even with breakthrough discoveries for some species made about 1975, the process still is not commercial. One major roadblock has been the difficulty in culturing anything but very young tissue. A preferred explant has been cotyledon or hypocotyl tissue from newly sprouted seeds. Workers have had minor success on some species, or on some genotypes within species, with trees up to 2-3 years of age. But the success has not been achieved with sufficient consistency for commercial application. Only with coast redwood, Sequoia sempervirens, can culture of trees from older trees be considered as successful [Jansson and Bornman, Physiol. Plant. 49:105-111 (1980)].
There are many advantages inherent in culturing tissue from older trees. A principal one is that it may take from five to 20 years before the phenotypic expression of an individual genotype can be adequately judged. This is as yet virtually an impossibility for younger trees. A number of investigators have recently summarized the state of the art in culturing older tissue and all point to the difficulty of the problems yet to be solved [Sommer and Brown in Plant Cell and Tissue Culture: Principles and Applications, Sharp et al., eds., pp. 461-491, Ohio State University Press (1979); Minocha, Can. J. Bot. 58:366-370 (1980); Karnosky, Bio Science 31(2):114-120 (1981)].
Despite the difficulties, some progress has been made in asexual reproduction of older wood. The work has been largely directed to various species of Pinus and two approaches might be noted. One done in vivo involves treating living trees with hormones to induce shoots from latent fascicular buds. A cytokinin is always present and other hormones may be used as well. Any shoots that form are excised and placed in a rooting mixture. This approach is exemplified in work reported by Whitehill and Schwabe, Physiol. Plant. 35:66-71 (1975); Cohen and Shanks, J. Amer. Soc. Hort. Sci. 100:404-406 (1975); Marino, Shahan, and Greenwood, Proc., Plant Growth Regulator Working Group, Fourth Annual Meeting (1977); and Cohen, J. Amer. Soc. Hort. Sci. 103:483-484 (1978). The success of this approach has been inconsistent but, in general, the percentage of shoots ultimately rooted has been low.
The second approach done in vitro involves excision of fascicles and placing them, at least initially, in a culture medium containing a cytokinin. This treatment also induces formation of the latent bud at the apex of the fascicle. The budded fascicles can then be placed in another medium to induce shoot growth. Finally the shoots are placed in a rooting medium or mixture. Recent examples of this approach are Mehra-Palta, Smeltzer, and Mott, TAPPI Forest Biology Wood Chemistry Conference, Madison, Wisc (1977): David, Isemukali and David, Comptes Rendus, Series D, 287:245-248 (1978); Franclet, David, David, and Boulay, Comptes Rendus, Series D, 290:927-930 (1980); and Franclet in Micropropagation d'arbres forestiers, AFOCEL: Etudes et Recherches, No. 12:3-18 (1979).
Because of the more rapid growth of young trees grown from seeds, an additional goal of asexual reproduction from mature trees has been rejuvenation. The first steps toward this goal are apparent in the work of the French investigators cited above. Yet even with the initial progress of the last few years, a wholly practical system for asexual reproduction from coniferous trees of the very large quantities of plants or plantlings suitable for mass forest regeneration has yet to be found, prior to the present invention.